EFFECTS OF THERMAL CUTTING ON STEELS BASIC INFORMATION

Fabrication of steel structures usually requires cutting of components by thermal cutting processes such as oxyfuel, air carbon arc, and plasma arc. Thermal cutting processes liberate a large quantity of heat in the kerf, which heats the newly generated cut surfaces to very high temperatures.

As the cutting torch moves away, the surrounding metal cools the cut surfaces rapidly and causes the formation of a heat-affected zone analogous to that of a weld. The depth of the heat-affected zone depends on the carbon and alloy content of the steel, the thickness of the piece, the preheat temperature, the cutting speed, and the postheat treatment.

In addition to the microstructural changes that occur in the heat-affected zone, the cut surface may exhibit a slightly higher carbon content than material below the surface. The detrimental properties of the thin layer can be improved significantly by using proper preheat, or postheat, or decreasing cutting speed, or any combination thereof.

The hardness of the thermally cut surface is the most important variable influencing the quality of the surface as measured by a bend test. Plate chemistry (carbon content), Charpy V-notch toughness, cutting speed, and plate temperature are also important.

Preheating the steel prior to cutting, and decreasing the cutting speed, reduce the temperature gradients induced by the cutting operation, thereby serving to (1) decrease the migration of carbon to the cut surface, (2) decrease the hardness of the cut surface, (3) reduce distortion, (4) reduce or give more favorable distribution to the thermally induced stresses, and (5) prevent the formation of quench or cooling cracks.

The need for preheating increases with increased carbon and alloy content of the steel, with increased thickness of the steel, and for cuts having geometries that act as high stress raisers. Most recommendations for minimum preheat temperatures are similar to those for welding.

The roughness of thermally cut surfaces is governed by many factors such as (1) uniformity of the preheat, (2) uniformity of the cutting velocity (speed and direction), and (3) quality of the steel. The larger the nonuniformity of these factors, the larger is the roughness of the cut surface. The roughness of a surface is important because notches and stress raisers can lead to fracture.

The acceptable roughness for thermally cut surfaces is governed by the job requirements and by the magnitude and fluctuation of the stresses for the particular component and the geometrical detail within the component. In general, the surface roughness requirements for bridge components are more stringent than for buildings.

The desired magnitude and uniformity of surface roughness can be achieved best by using automated thermal cutting equipment where cutting speed and direction are easily controlled. Manual procedures tend to produce a greater surface roughness that may be unacceptable for primary tension components. This is attributed to the difficulty in controlling both the cutting speed and the small transverse perturbations from the cutting direction.

DIFFERENT TYPES OF STAINLESS STEELS BASIC INFORMATION AND TUTORIALS

Iron-base alloys containing between 11% and 30% chromium form a tenacious and highly protective chrome oxide layer that gives these alloys excellent corrosion-resistant properties. There are a great number of alloys that are generally referred to as stainless steels, and they fall into three general classifications.

Austenitic stainless steels contain usually 8% to 12% nickel, which stabilizes the austenitic phase. These are the most popular of the stainless steels. With 18% to 20% chromium, they have the best corrosion resistance and are very tough and can undergo severe forming operations.

These alloys are susceptible to embrittlement when heated in the range of 593 to 816°C. At these temperatures, carbides precipitate at the austenite grain boundaries, causing a local depletion of the chromium content in the adjacent region, so this region loses its corrosion resistance.

Use of “extra low carbon” grades and grades containing stabilizing additions of strong carbide-forming elements such as niobium minimizes this problem. These alloys are also susceptible to stress corrosion in the presence of chloride environments.

Ferritic stainless steels are basically straight Fe-Cr alloys. Chromium in excess of 14% stabilizes the low-temperature ferrite phase all the way to the melting point. Since these alloys do not undergo a phase change, they cannot be hardened by heat treatment. They are the least expensive of the stainless alloys.

Martensitic stainless steels contain around 12% Cr. They are austenitic at elevated temperatures but ferritic at low; hence they can be hardened by heat treatment.

To obtain a significant response to heat treatment, they have higher carbon contents than the other stainless alloys. Martensitic alloys are used for tools, machine parts, cutting instruments, and other applications requiring high strength. The austenitic alloys are nonmagnetic, but the ferritic and martensitic grades are ferromagnetic.

TYPES OF HEAT TREATMENT OF STEEL BASIC INFORMATION AND TUTORIALS

The properties of steels can be greatly modified by thermal treatments, which change the internal crystalline structure of the alloy. Hardening of steel is based on the fact that iron undergoes a change in crystal structure when heated above its “critical” temperature.

Above this critical transformation temperature, the structure is called austenite, a phase capable of dissolving carbon up to 2%. Below the critical temperature, the steel transforms to ferrite, in which carbon is insoluble and precipitates as an iron carbide compound, FeeC (sometimes called cementite).

If a steel is cooled rapidly from above the critical temperature, the carbon is unable to diffuse to form cementite, and the austenite transforms instead to an extremely hard metastable constituent called martensite, in which the carbon is held in supersaturation. The hardness of the martensite depends sensitively on the carbon content.

Low-carbon steels (below about 0.20%) are seldom quenched, while steels above about 0.80% carbon are brittle and liable to crack on quenching. Plain carbon steels must be quenched at very fast rates in order to be hardened. Alloying elements can be added to decrease the necessary cooling rates to cause hardening; some alloy steels will harden when cooled in air from above the critical temperature.

It should be noted, however, that it is the amount of carbon that primarily determines the properties of the alloy; the alloying elements serve to make the response to heat treatment possible.

Normalizing is a treatment in which the steel is heated over the critical temperature and allowed to cool in still air. The purpose of normalizing is to homogenize the steel. The carbon in the steel will appear as a fine lamellar product of cementite and ferrite called pearlite.

Annealing is similar to normalizing, except the steel is very slowly cooled from above the critical. The carbides are now coarsely divided and the steel is in its softest state, as may be desired for cold-forming or machining operations.

Process annealing is a treatment carried out below the critical temperature designed to recrystallize the ferrite following a cold-working operation. Metals become hardened and embrittled by plastic deformation, but the original state can be restored if the alloy is heated high enough to cause new strain-free grains to nucleate and replace the prior strained structure. This treatment is commonly applied as a final processing for low-carbon steels where ductility and toughness are important, or as an intermediate treatment for such products as wire that are formed by cold working.

Stress-relief annealing is a thermal treatment carried out at a still lower temperature. No structural changes take place, but its purpose is to reduce residual stresses that may have been introduced by previous nonuniform deformation or heating.

Tempering is a treatment that always follows a hardening (quenching) treatment. After hardening, steels are extremely hard, but relatively weak owing to their brittleness. When reheated to temperatures below the critical, the martensitic structure is gradually converted to a ferrite-carbide aggregate that optimizes strength and toughness.

When steels are tempered at about 260°C, a particularly brittle configuration of precipitated carbides forms; steels should be tempered above or below this range. Another phenomenon causing embrittlement occurs in steels particularly containing chromium and manganese that are given a tempering cycle that includes holding at or cooling through temperatures around 567 to 621°C. Small molybdenum additions retard this effect, called temper brittleness. It is believed to be caused by a segregation of trace impurity elements to the grain boundaries.

CLASSIFICATION OF FERROUS MATERIALS BASIC INFORMATION AND TUTORIALS

Iron and steel may be classified on the basis of composition, use, shape, method of manufacture, etc. Some of the more important ferrous alloys are described in the sections below.

Ingot iron is commercially pure iron and contains a maximum of 0.15% total impurities. It is very soft and ductile and can undergo severe cold-forming operations. It has a wide variety of applications based on its formability.

Its purity results in good corrosion resistance and electrical properties, and many applications are based on these features. The average tensile properties of Armco ingot iron plates are tensile strength 320 MPa (46,000 lb/in2); yield point 220 MPa (32,000 lb/in2); elongation in 8 in, 30%; Young’s modulus 200 GPa (29 # 106 lb/in2).

Plain carbon steels are alloys of iron and carbon containing small amounts of manganese (up to 1.65%) and silicon (up to 0.50%) in addition to impurities of phosphorus and sulfur. Additions up to 0.30% copper may be made in order to improve corrosion resistance.

The carbon content may range from 0.05% to 2%, although few alloys contain more than 1.0%, and the great bulk of steel tonnage contains from 0.08% to 0.20% and is used for structural applications.

Medium-carbon steels contain around 0.40% carbon and are used for constructional purposes—tools, machine parts, etc. High-carbon steels have 0.75% carbon or more and may be used for wear and abrasion-resistance applications such as tools, dies, and rails.

Strength and hardness increase in proportion to the carbon content while ductility decreases. Phosphorus has a significant hardening effect in low-carbon steels, while the other components have relatively minor effects within the limits they are found.

It is difficult to generalize the properties of steels, however, since they can be greatly modified by cold working or heat treatment.

High-strength low-alloy steels are low-carbon steels (0.10% to 0.15%) to which alloying elements such as phosphorus, nickel, chromium, vanadium, and niobium have been added to obtain higher strength.

This class of steel was developed primarily by the transportation industry to decrease vehicle weight, but the steels are widely used. Since thinner sections are used, corrosion resistance is more important, and copper is added for this purpose.

STEEL SHEET PILING APPLICATIONS BASIC INFORMATION AND TUTORIALS

Steel sheet piling is used in many types of temporary works and permanent structures. The sections are designed to provide the maximum strength and durability at the lowest possible weight consistent with good driving qualities. The design of the section interlocks facilitates pitching and driving and results in a continuous wall with a series of closely fitting joints.

A comprehensive range of sections in both Z and U forms with a wide range of sizes and weights is  obtainable in various different grades of steel which enables the most economic choice to be made to suit the nature and requirements of any given contract.

For applications where corrosion is an issue, sections with minimum thickness can be delivered to maximise the effective life of the structure. The usual requirements for minimum overall thickness of 10 mm, 12 mm or 1/2 inch can be met. Corner and junction piles are available to suit all requirements.

Typical Uses

River control structures and flood defence

Steel sheet piling has traditionally been used for the support and protection of river banks, lock and sluice construction, and flood protection. Ease of use, length of life and the ability to be driven through water make piles the obvious choice.

Ports and harbours

Steel sheet piling is a tried and tested material to construct quay walls speedily and economically. Steel sheet piles can be designed to cater for heavy vertical loads and large bending moments.

Pumping stations

Historically used as temporary support for the construction of pumping stations, sheet piling can be easily designed as the permanent structure with substantial savings in time and cost. Although pumping stations tend to be rectangular, circular construction should be considered as advantages can be gained from the resulting open structure.

Bridge abutments

Abutments formed from sheet piles are most cost effective in situations when a piled foundation is required to support the bridge or where speed of construction is critical. Sheet piling can act as both foundation and abutment and can be driven in a single operation, requiring a minimum of space and time for construction.

Road widening retaining walls

Key requirements in road widening include minimised land take and speed of construction  particularly in lane rental situations. Steel sheet piling provides these and eliminates the need for soil excavation and disposal.

STEEL JOIST FLOORS BASIC INFORMATION AND TUTORIALS

The lightest floor system in common use is the open-web steel joist construction. It is popular for all types of light occupancies, principally because of initial low cost.

Many types of open-web joists are available. Some employ bars in their makeup, while others are entirely of rolled shapes; they all conform to standards and good practice specifications promulgated by the Steel Joist Institute and the American Institute of Steel Construction.

All joists conform to the standard loading tables and carry the same size designation so that designers need only indicate on project drawings the standard marking without reference to manufacturer, just as for a steel beam or column section.

Satisfactory joists construction is assured by adhering to SJI and AISC recommendations. Joists generally are spaced 2 ft c to c. They should be adequately braced (with bridging) during construction to prevent rotation or buckling, and to avoid ‘‘springy’’ floors, they should be carefully selected to provide sufficient depth.

This system has many advantages: Falsework is eliminated. Joists are easily handled, erected, and connected to supporting beams—usually by tack welding.

Temporary coverage and working platforms are quickly placed. The open space between joists, and through the webs, may be utilized for ducts, cables, light fixtures, and piping. A thin floor slab may be cast on steel lath, corrugated-steel sheets, or wire-reinforced paper lath laid on top of the joists. A plaster ceiling may be suspended or attached directly to the bottom flange of the joists.

Lightweight beams, or so-called ‘‘junior’’ beams, are also used in the same manner as open-web joists, and with the same advantages and economy, except that the solid webs do not allow as much freedom in installation of utilities.

Beams may be spaced according to their safe load capacity; 3- and 4-ft spacings are common. As a type, therefore, the lightweight-steel-beam floor is intermediate between concrete arches and open-web joists.

STRUCTURAL STEEL FABRICATION BASIC INFORMATION

When considering fabrication, as well as erection of the fabricated product, the designer must taken into account contractual matters, work by others on the construction team, schedule implications of the design, and quality assurance matters.

Fortunately, there are well established aids for these considerations. Contractual questions such as what constitutes structural steel, procedures for preparing and approving the shop detail drawings, and standard fabrication procedures and tolerances are all addressed in the AISC’s Code of Standard Practice.

Insights on economical connection details and the impact of material selection on mill material deliveries are generally available from the fabricator’s engineering staff. These engineers are also able to comment on unique erection questions.

Quality assurance questions fall into two categories, fabrication operations and field operations. Today, sound quality control procedures are in place in most fabrication shops through an AISC program which prequalifies fabricators.

There are three levels of qualification: I, II and III, with Level III being the most demanding. Fabricators with either a Level I or Level II certification are suitable for almost all building work. Most engineers incorporate the AISC’s Code of Standard Practice in their project specification.

Shop Detail Drawings

Detail drawings are prepared by the fabricator to delineate to his work force the fabrication requirements. Because each shop has certain differences in equipment and/or procedures, the fabricator develops details which, when matched with his processes, are the most economical.

To accomplish this end, the design drawings need to be complete, showing all structural steel requirements, and should include design information on the forces acting at connections. Designers should avoid specifying deck openings and beam penetrations through notes on the drawings. This is a frequent cause of extra costs on fabrication contracts.

Fabrication Processes

Mill material is cut to length by sawing, shearing, or flame cutting. Columns may also be milled to their final length. Holes for fasteners are drilled or punched. Punched and reamed holes are seldom used in building construction. Cuts for weld preparation, web openings, and dimensional clearances are flame cut.

AISC guidelines for each of these processes are associated with the AISC’s fabricator prequalification program. Welding for building construction is performed in accordance with the provisions of the AWS Structural Welding Code, D1.1. Most requirements can be satisfied using pre-qualified welding procedures.

BENDING AND WELDING LIMITATIONS OF REBARS BASIC AND TUTORIALS

The ACI 318 Building Code contains the following restrictions:

1. All bars must be bent without heating, except as permitted by the engineer.

2. Bars partly embedded in hardened concrete may not be bent without permission of the engineer.

3. No welding of crossing bars (tack welding) is permitted without the approval of the engineer.

4. For unusual bends, heating may be permitted because bars bend more easily when heated.

If not embedded in thin sections of concrete, heating the bars to a maximum temperature of 1500 F facilitates bending, usually without damage to the bars or splitting of the concrete.

If partly embedded bars are to be bent, heating controlled within these limits, plus the provision of a round fulcrum for the bend to avoid a sharp kink in the bar, are essential.

Tack welding creates a metallurgical notch effect, seriously weakening the bars. If different size bars are tacked together, the notch effect is aggravated in the larger bar.

Tack welding therefore should never be permitted at a point where bars are to be fully stressed, and never for the assembly of ties or spirals to column verticals or stirrups to main beam bars.

When large, preassembled reinforcement units are desired, the engineer can plan the tack welding necessary as a supplement to wire ties at points of low stress or to added bars not required in the design.

STEEL STRUCTURES ERECTION EQUIPMENT CIVIL ENGINEERING TUTORIALS

STEEL STRUCTURES ERECTION EQUIPMENT TUTORIALS
What Are The Steel Structure Erection Equipment?

If there is a universal piece of erection equipment, it is the crane. Mounted on wheels or tractor threads, it is extremely mobile, both on the job and in moving from job to job.

Practically all buildings are erected with this efficient raising device. The exception, of course, is the skyscraper whose height exceeds the reach of the crane.  Operating on ground level, cranes have been used to erect buildings of about 20 stories, the maximum height being dependent on the length of the boom and width of building.


The guy derrick is a widely used raising device for erection of tall buildings. Its principal asset is the ease by which it may be ‘‘jumped’’ from tier to tier as erection proceeds upward. The boom and mast reverse position; each in turn serves to lift up the other.

It requires about 2 h to make a two-story jump. Stiff-leg derricks and gin poles are two other rigs sometimes used, usually in the role of auxiliaries to cranes or guy derricks. Gin poles are the most elementary— simply a guyed boom.

The base must be secure because of the danger of kicking out. The device is useful for the raising of incidental materials, for dismantling and lowering of larger rigs, and for erection of steel on light construction where the services of a crane are unwarranted.

Stiff-leg derricks are most efficient where they may be set up to remain for long periods of time. They have been used to erect multistory buildings but are not in popular favor because of the long time required to jump from tier to tier.

Among the principal uses for stiff legs are (1) unloading steel from railroad cars for transfer to trucks, (2) storage and sorting, and (3) when placed on a flat roof, raising steel to roof level, where it may be sorted and placed within each of a guy derrick.

Less time for ‘‘jumping’’ the raising equipment is needed for cranes mounted on steel box-type towers, about three stories high, that are seated on interior elevator wells or similar shafts for erecting steel.

These tower cranes are simply jacked upward hydraulically or raised by cables, with the previously erected steel-work serving as supports. In another method, a stiff-leg derrick is mounted on a trussed platform, spanning two or more columns, and so powered that it can creep up the erected exterior columns.

In addition to the advantage of faster jumps, these methods permit steel erection to proceed as soon as the higher working level is reached.

STEEL MAKING METHODS BASICS AND TUTORIALS

STEEL MAKING METHODS BASIC INFORMATION
What Are The Basic Steel Making Techniques And Methods?


Structural steel is usually produced today by one of two production processes. In the traditional process, iron or ‘‘hot metal’’ is produced in a blast furnace and then further processed in a basic oxygen furnace to make the steel for the desired products.

Alternatively, steel can be made in an electric arc furnace that is charged mainly with steel scrap instead of hot metal. In either case, the steel must be produced so that undesirable elements are reduced to levels allowed by pertinent specifications to minimize adverse effects on properties.

In a blast furnace, iron ore, coke, and flux (limestone and dolomite) are charged into the top of a large refractory-lined furnace. Heated air is blown in at the bottom and passed up through the bed of raw materials.

A supplemental fuel such as gas, oil, or powdered coal is also usually charged. The iron is reduced to metallic iron and melted; then it is drawn off periodically through tap holes into transfer ladles.

At this point, the molten iron includes several other elements (manganese, sulfur, phosphorus, and silicon) in amounts greater than permitted for steel, and thus further processing is required.

In a basic oxygen furnace, the charge consists of hot metal from the blast furnace and steel scrap. Oxygen, introduced by a jet blown into the molten metal, reacts with the impurities present to facilitate the removal or reduction in level of unwanted elements, which are trapped in the slag or in the gases produced.

Also, various fluxes are added to reduce the sulfur and phosphorus contents to desired levels. In this batch process, large heats of steel may be produced in less than an hour.

An electric-arc furnace does not require a hot metal charge but relies mainly on steel scrap. The metal is heated by an electric arc between large carbon electrodes that project through the furnace roof into the charge.

Oxygen is injected to speed the process. This is a versatile batch process that can be adapted to producing small heats where various steel grades are required, but it also can be used to produce large heats.

Ladle treatment is an integral part of most steelmaking processes. The ladle receives the product of the steel making furnace so that it can be moved and poured into either ingot molds or a continuous casting machine.

While in the ladle, the chemical composition of the steel is checked, and alloying elements are added as required. Also, deoxidizers are added to remove dissolved oxygen. Processing can be done at this stage to reduce further sulfur content, remove undesirable nonmetallics, and change the shape of remaining inclusions.

Thus significant improvements can be made in the toughness, transverse properties, and through-thickness ductility of the finished product. Vacuum degassing, argon bubbling, induction stirring, and the injection of rare earth metals are some of the many procedures that may be employed.

Killed steels usually are deoxidized by additions to both furnace and ladle. Generally, silicon compounds are added to the furnace to lower the oxygen content of the liquid metal and stop oxidation of carbon (block the heat).

This also permits addition of alloying elements that are susceptible to oxidation. Silicon or other deoxidizers, such as aluminum, vanadium, and titanium, may be added to the ladle to complete deoxidation.

Aluminum, vanadium, and titanium have the additional beneficial effect of inhibiting grain growth when the steel is normalized. (In the hot-rolled conditions, such steels have about the same ferrite grain size as semikilled steels.)

Killed steels deoxidized with aluminum and silicon (made to finegrain practice) often are used for structural applications because of better notch toughness and lower transition temperatures than semikilled steels of the same composition.

(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron and Steel Engineers, Pittsburgh, Pa.)

SHANLEY'S THEORY OF INELASTIC BUCKLING OF STEEL MEMBERS BASIC AND TUTORIALS

SHANLEY'S THEORY OF INELASTIC BUCKLING OF STEEL MEMBERS BASIC INFORMATION
What Is Shanley's Theory of Inelastic Buckling Of Steel Members?



Although the tangent modulus theory appears to be invalid for inelastic materials, careful experiments have shown that it leads to more accurate predictions than the apparently rigorous reduced modulus theory.

This paradox was resolved by Shanley [1], who reasoned that the tangent modulus theory is valid when buckling is accompanied by a simultaneous increase in the applied load (see Figure 3.8) of sufficient magnitude to prevent strain reversal in the member.

When this happens, all the bending stresses and strains are related by the tangent modulus of elasticity Et , the initial modulus E does not feature, and so the buckling load is equal to the tangent modulus value Ncr,t .

As the lateral deflection of the member increases as shown in Figure 3.8, the tangent modulus Et decreases (see Figure 3.6b) because of the increased axial and bending strains, and the post-buckling curve approaches a maximum load Nmax which defines the ultimate resistance of the member.

Also shown in Figure 3.8 is a post-buckling curve which commences at the reduced modulus load Ncr,r (at which buckling can take place without any increase in the load). The tangent modulus load Ncr,t is the lowest load at which buckling can begin, and the reduced modulus load Ncr,r is the highest load for which the member can remain straight.

It is theoretically possible for buckling to begin at any load between Ncr,t and Ncr,r . It can be seen that not only is the tangent modulus load more easily calculated, but it also provides a conservative estimate of the member resistance, and is in closer agreement with experimental results than the reduced modulus load.

For these reasons, the tangent modulus theory of inelastic buckling has gained wide acceptance.

HANDBOOK OF STRUCTURAL STEEL CONNECTION DESIGN AND DETAILS FREE EBOOK DOWNLOAD LINK

HANDBOOK OF STRUCTURAL STEEL CONNECTION DESIGN AND DETAILS FREE EBOOK
Free E-Book Download Link: Handbook of Structural Steel Connection Design and Details

Handbook of Structural Steel Connection Design and Details Editorial Reviews


This book not not only gives you the best and latest methods in connection design, it supplies fabricated examples on the CD-ROM that you can use for instant application and configuration of your own designs.

Featuring a broad range of design methods and details, the Handbook demonstrates the newest techniques and materials in welded joint design and production...seismically resistant connnections...partially restrained connections...steel decks...inspection and quality control...and more.

You get the newest connection designs based on load and resistance factor AISC design methods; special methods for seismic connection design; new material on fracture and fatigue design; improved methods of connection force analysis for various structures; 400 illustrations that show you how to do the job right; and much more.

Book Description
Publication Date: April 15, 1999 | ISBN-10: 0070614970 | ISBN-13: 978-0070614970 | Edition: 1

About the Author
Akbar R. Tamboli is a senior project engineer with CUH2A in Princeton, New Jersey. He was previously vice president and project manager with Irwin G. Cantor, P.E., Consulting Engineers in New York City. A Fellow of the American Society of Civil Engineers, Mr. Tamboli is the editor of Steel Design Handbook: LRFD Method, published by McGraw-Hill.

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EFFECTS OF THERMAL CUTTING ON STRUCTURAL STEELS BASICS AND TUTORIALS

STRUCTURAL STEELS THERMAL CUTTING EFFECTS BASIC INFORMATION
What Are The Thermal Cutting Effects On Structural Steel?


Fabrication of steel structures usually requires cutting of components by thermal cutting processes such as oxyfuel, air carbon arc, and plasma arc. Thermal cutting processes liberate a large quantity of heat in the kerf, which heats the newly generated cut surfaces to very high temperatures.

As the cutting torch moves away, the surrounding metal cools the cut surfaces rapidly and causes the formation of a heat-affected zone analogous to that of a weld. The depth of the heat-affected zone depends on the carbon and alloy content of the steel, the thickness of the piece, the preheat temperature, the cutting speed, and the postheat treatment.

In addition to the microstructural changes that occur in the heat-affected zone, the cut surface may exhibit a slightly higher carbon content than material below the surface. The detrimental properties of the thin layer can be improved significantly by using proper preheat, or postheat, or decreasing cutting speed, or any combination thereof.

The hardness of the thermally cut surface is the most important variable influencing the quality of the surface as measured by a bend test. Plate chemistry (carbon content), Charpy V-notch toughness, cutting speed, and plate temperature are also important.

Preheating the steel prior to cutting, and decreasing the cutting speed, reduce the temperature gradients induced by the cutting operation, thereby serving to (1) decrease the migration of carbon to the cut surface, (2) decrease the hardness of the cut surface, (3) reduce distortion, (4) reduce or give more favorable distribution to the thermally induced stresses, and (5) prevent the formation of quench or cooling cracks.

The need for preheating increases with increased carbon and alloy content of the steel, with increased thickness of the steel, and for cuts having geometries that act as high stress raisers. Most recommendations for minimum preheat temperatures are similar to those for welding.

The roughness of thermally cut surfaces is governed by many factors such as (1) uniformity of the preheat, (2) uniformity of the cutting velocity (speed and direction), and (3) quality of the steel. The larger the nonuniformity of these factors, the larger is the roughness of the cut surface.

The roughness of a surface is important because notches and stress raisers can lead to fracture. The acceptable roughness for thermally cut surfaces is governed by the job requirements and by the magnitude and fluctuation of the stresses for the particular component and the geometrical detail within the component.

In general, the surface roughness requirements for bridge components are more stringent than for buildings. The desired magnitude and uniformity of surface roughness can be achieved best by using automated thermal cutting equipment where cutting speed and direction are easily controlled.

Manual procedures tend to produce a greater surface roughness that may be unacceptable for primary tension components. This is attributed to the difficulty in controlling both the cutting speed and the small transverse perturbations from the cutting direction.

(R. L. Brockenbrough and J. M. Barsom, Metallurgy, Chapter 1.1 in Constructional Steel Design—An International Guide, R. Bjorhovde et al, Eds., Elsevier Science Publishers, Ltd., New York.)

COLD FORMED STEEL SHAPES BASICS AND TUTORIALS

COLD FORMED STEEL SHAPES BASIC INFORMATION
What Are Cold Formed Steel Shapes?


A wide variety of shapes can be produced by cold-forming and manufacturers have developed a wide range of products to meet specific applications. Figure 3.11 shows the common shapes of typical cold-formed steel framing members. Figure 3.12 shows common shapes for profiled sheets and trays used for roofing and wall cladding and for load bearing deck panels.

For common applications, such as structural studs, industry organizations, such as the Steel Framing Alliance (SFA) and the Steel Stud Manufacturers Association (SSMA) have developed standard shapes and nomenclature to promote uniformity of product availability across the industry. Figure 3.11 shows the generic shapes covered by the Universal Designator System.

The designator consists of four sequential codes. The first code is a three or four-digit number indicating the member web depth in 1/100 inches. The second is a single letter indicating the type of member, as follows:


framing member with stiffening lips
L = Angle or L-header
F = Furring channels
U = Cold-rolled channel
T = Track section


The third is a three-digit numeral indication flange width in 1/100 inches followed by a dash. The fourth is a two or three-digit numeral indicating the base steel thickness in 1/1000 inch (mils). As an example, the designator system for a 6'', C-shape with 1-5/8'' (1.62'') flanges and made with 0.054'' thick steel is 600S162-54.


Special Design Considerations for Cold-Formed Steel
Structural design of cold-formed members is in many respects more challenging than the design of hot rolled, relatively thick, structural members. A primary difference is cold-formed members are more susceptible to buckling due to their limited thickness.

The fact that the yield strength of the steel is increased in the cold-forming process creates a dilemma for the designer. Ignoring the increased strength is conservative, but results in larger members, hence more costly, than is needed if the increased yield strength is considered.

Corrosion creates a greater percent loss of cross section than is the case for thick members. All cold-formed steel members are coated to protect steel from corrosion during the storage and transportation phases of construction as well as for the life of the product.

Because of its effectiveness, hot-dipped zinc galvanizing is most commonly used. Structural and non-structural framing members are required to have a minimum metallic coating that complies with ASTM A1003/A1003M, as follows:
■ structural members – G60 and
■ non-structural members G40 or equivalent minimum.

To prevent galvanic corrosion special care is needed to isolate the cold-formed members from dissimilar metals, such as copper. The design, manufacture and use of cold-formed steel framing is governed by standards that are developed and maintained by the American Iron and Steel Institute along with organizations such as ASTM, and referenced in the building codes.

GENERAL APPROACHES TO FABRICATION AND ERECTION OF BRIDGE STEELWORKS BASICS AND TUTORIALS

FABRICATION AND ERECTION OF BRIDGE STEELWORKS GENERAL APPROACHES
What Are The General Approaches To Fabrication and Erection Of Bridge Steelworks

The objective in steel bridge construction is to fabricate and erect the structure so that it will have the geometry and stressing designated on the design plans, under full dead load at normal temperature.

This geometry is known as the geometric outline.

In the case of steel bridges there have been, over the decades, two general procedures for achieving this objective:

1. The “field adjustment” procedure — Carry out a continuing program of steelwork surveys and measurements in the field as erection progresses, in an attempt to discover fabrication and erection deficiencies; and perform continuing steelwork adjustments in an effort to compensate for such deficiencies and for errors in span baselines and pier elevations.

2. The “shop control” procedure — Place total reliance on first-order surveying of span baselines and pier elevations, and on accurate steelwork fabrication and erection augmented by meticulous construction engineering; and proceed with erection without any field adjustments, on the basis that the resulting bridge deadload geometry and stressing will be as good as can possibly be achieved.

Bridge designers have a strong tendency to overestimate the capability of field forces to accomplish accurate measurements and effective adjustments of the partially erected structure, and at the same time they tend to underestimate the positive effects of precise steel bridgework fabrication and erection.

As a result, we continue to find contract drawings for major steel bridges that call for field evolutions such as the following:

1. Continuous trusses and girders

— At the designated stages, measure or “weigh” the reactions on each pier, compare them with calculated theoretical values, and add or remove bearing-shoe shims to bring measured values into agreement with calculated values.

2. Arch bridges

— With the arch ribs erected to midspan and only the short, closing “crown sections” not yet in place, measure thrust and moment at the crown, compare them with calculated theoretical values, and then adjust the shape of the closing sections to correct for errors in span-length measurements and in bearing-surface angles at skewback supports, along with accumulated fabrication and erection errors.

3. Suspension bridges

— Following erection of the first cable wire or strand across the spans from anchorage to anchorage, survey its sag in each span and adjust these sags to agree with calculated theoretical values.

4. Arch bridges and suspension bridges — Carry out a deck-profile survey along each side of the bridge under the steel-load-only condition, compare survey results with the theoretical profile, and shim the suspender sockets so as to render the bridge floor beams level in the completed structure.

5. Cable-stayed bridges

— At each deck-steelwork erection stage, adjust tensions in the newly erected cable stays so as to bring the surveyed deck profile and measured stay tensions into agreement with calculated theoretical data.

There are two prime obstacles to the success of “field adjustment” procedures of whatever type: (1) field determination of the actual geometric and stress conditions of the partially erected structure and its components will not necessarily be definitive, and (2) calculation of the corresponding “proper” or “target” theoretical geometric and stress conditions will most likely prove to be less than authoritative.

STEEL FABRICATION PROCESSES BASICS AND TUTORIALS

STEEL FABRICATION BASIC PROCESSES
What Is Steel Fabrication?

When considering fabrication, as well as erection of the fabricated product, the designer must taken into account contractual matters, work by others on the construction team, schedule implications of the design, and quality assurance matters.

Fortunately, there are well established aids for these considerations. Contractual questions such as what constitutes structural steel, procedures for preparing and approving the shop detail drawings, and standard fabrication procedures and tolerances are all addressed in the AISC’s Code of Standard Practice.

Insights on economical connection details and the impact of material selection on mill material deliveries are generally available from the fabricator’s engineering staff. These engineers are also able to comment on unique erection questions.

Quality assurance questions fall into two categories, fabrication operations and field operations. Today, sound quality control procedures are in place in most fabrication shops through an AISC program which prequalifies fabricators.

There are three levels of qualification: I, II and III, with Level III being the most demanding. Fabricators with either a Level I or Level II certification are suitable for almost all building work. Most engineers incorporate the AISC’s Code of Standard Practice in their project specification.

Shop Detail Drawings

Detail drawings are prepared by the fabricator to delineate to his work force the fabrication requirements. Because each shop has certain differences in equipment and/or procedures, the fabricator develops details which, when matched with his processes, are the most economical.

To accomplish this end, the design drawings need to be complete, showing all structural steel requirements, and should include design information on the forces acting at connections.

Designers should avoid specifying deck openings and beam penetrations through notes on the drawings. This is a frequent cause of extra costs on fabrication contracts.

Fabrication Processes

Mill material is cut to length by sawing, shearing, or flame cutting. Columns may also be milled to their final length. Holes for fasteners are drilled or punched.

Punched and reamed holes are seldom used in building construction. Cuts for weld preparation, web openings, and dimensional clearances are flame cut. AISC guidelines for each of these processes are associated with the AISC’s fabricator prequalification program.

Welding for building construction is performed in accordance with the provisions of the AWS Structural Welding Code, D1.1. Most requirements can be satisfied using pre-qualified welding procedures.

STEEL PLATES MECHANICAL PROPERTIES AND MANUFACTURING PROCESS BASICS AND TUTORIALS

STEEL PLATES BASIC INFORMATION - MECHANICAL PROPERTIES AND MANUFACTURING
What Are Steel Plates? What Are The Mechanical Properties Of Steel Plates?


This article covers hot-rolled uncoated steel plates with a minimum thickness of 3 mm, supplied flat or precurved
in any shape as required. Steel for cold forming is not within the scope of this article.


Manufacturing process
Rimming steel shall not be allowed and the steel shall be at least semi-killed in the deoxidation process. The plates may be produced directly on reversing mill, by cutting from parent plates rolled on reversing mill or hot rolled wide strips.

The plate edges may be as rolled or sheared, flame cut or chamfered. The products may be supplied in as rolled, normalized or quenched and tempered condition, or with controlled rolling (normalized rolling or thermo-mechanical rolling).


Strength
The nominal yield strength shall be in the range of 235 N/mm2 to 690 N/mm2. The nominal tensile strength shall be in the range of 300 N/mm2 to 1000 N/mm2.

Ductility
The elongation after fracture on proportional gauge length shall be at least 15 %, for nominal yield strength not greater than 460 N/mm2; and shall be at least 10 % for nominal yield strength greater than 460 N/mm2. The tensile strength to yield strength ratio shall be at least 1.2 based on nominal values, or at least 1.1 based on actual values, for nominal yield strength not greater than 460 N/mm2.

NOTE Conversion of elongation values measured not based on proportional gauge length is necessary and shall be performed according to BS EN ISO 2566-1.

Impact toughness
As a minimum, the product shall be able to absorb at least 27 J of impact energy at 20 °C. NOTE Depending on other factors including the thickness and minimum service temperature, the impact toughness should also conform to the appropriate requirements as given in BS 5950-1.

Through thickness deformation properties
Where appropriate, through thickness deformation properties shall be specified to guarantee adequate deformation capacity perpendicular to the surface to provide ductility and toughness against lamellar tearing.


Chemical composition
In general, based on ladle analysis, carbon content shall not exceed 0.26 %; maximum CEV and content of impurities shall be in accordance with the requirements given in Table 1.

NOTE 1 Interpolation for maximum content shall be allowed for design strength not given in Table 1.
NOTE 2 Depending on the product thickness or variation in metallurgical process and intended use, the requirements for chemical composition might vary and shall be referred to BS EN 10025-1, BS EN 10025 2, BS EN 10025-3, BS EN 10025-4, BS EN 10025-5 and BS EN 10025-6.

Table 1 — Chemical composition requirements for steel plates based on ladle analysis

Dimensional and mass tolerances

Dimensions

In general, the deviation in actual thickness from nominal plate thickness shall not exceed the larger of ± 2 mm and ± 10 %.

Mass

In general, the deviation in actual mass from mass computed using a density of 7850 kg/m3 shall be limited by the dimensional tolerances.

STEEL ALLOYS DIFFERENT TYPES BASICS AND TUTORIALS

DIFFERENT TYPE OF STEEL ALLOYS BASIC INFORMATION
What Are The Different Types of Steel Alloys?


Alloy metals can be used to alter the characteristics of steel. By some counts, there are as many as 250,000 different alloys of steel produced. Of these, as many as 200 may be used for civil engineering applications.

Rather than go into the specific characteristics of selected alloys, the general effect of different alloying agents will be presented. Alloy agents are added to improve one or more of the following properties:

1. hardenability
2. corrosion resistance
3. machinability
4. ductility
5. strength

Common alloy agents, their typical percentage range, and their effects are summarized
in Table 3.1.

By altering the carbon and alloy content and by using different heat treatments, steel can be produced with a wide variety of characteristics. These are classified as follows:

1. Low alloy
■ Low carbon
   Plain
   High strength–low alloy
■ Medium carbon
   Plain
   Heat treatable
■ High carbon
   Plain
   Tool

2. High Alloy
■ Tool
■ Stainless

Steels used for construction projects are predominantly low- and medium-carbon plain steels. Stainless steel has been used in some highly corrosive applications, such as dowel bars in concrete pavements and steel components in swimming pools and drainage lines.

The Specialty Steel Industry of North America, SSINA, promotes the use of stainless steel for structural members where corrosion resistance is an important design consideration (SSINA, 1999).

The use and control of alloying agents is one of the most significant factors in the development of steels with better performance characteristics. The earliest specification for steel used in building and bridge construction, published in 1900, did not contain any chemical requirements.

In 1991 ASTM published the specification which controls content of 10 alloying elements in addition to carbon (Hassett, 2003).

HEAT TREATMENT OF STEELS BASICS AND TUTORIALS

STEELS HEAT TREATMENT BASIC INFORMATION
What Are The Different Heat Treatment of Steels?


Properties of steel can be altered by applying a variety of heat treatments. For example, steel can be hardened or softened by using heat treatment; the response of steel to heat treatment depends upon its alloy composition.

Common heat treatments employed for steel include annealing, normalizing, hardening, and tempering. The basic process is to heat the steel to a specific temperature, hold the temperature for a specified period of
time, then cool the material at a specified rate.

The temperatures used for each of the treatment types are shown in Figure 3.7.

Annealing
The objectives of annealing are to refine the grain, soften the steel, remove internal stresses, remove gases, increase ductility and toughness, and change electrical and magnetic properties. Four types of annealing can be performed, depending on the desired results of the heat treatment:

Full annealing requires heating the steel to about 50°C above the austenitic temperature line and holding the temperature until all the steel transforms into either austenite or austenite–cementite, depending on the carbon content. The steel is then cooled at a rate of about 20°C per hour in a furnace to a temperature of about 680°C, followed by natural convection cooling to room temperature.

Due to the slow cooling rate, the grain structure is a coarse pearlite with ferrite or cementite, depending on the carbon content. The slow cooling rate ensures uniform properties of the treated steel. The steel is soft and ductile.

Process annealing is used to treat work-hardened parts made with low carbon steel (i.e., less than 0.25 percent carbon). The material is heated to about 700°C and held long enough to allow recrystallization of the ferrite phase.

By keeping the temperature below 727°C, there is not a phase shift between ferrite and austenite, as occurs during full annealing. Hence, the only change that occurs is refinement of the size, shape, and distribution of the grain structure.

Stress relief annealing is used to reduce residual stresses in cast, welded, and cold-worked parts and cold-formed parts. The material is heated to 600 to 650°C, held at temperature for about one hour, and then slowly cooled in still air.

Spheroidization is an annealing process used to improve the ability of high carbon (i.e., more than 0.6 percent carbon) steel to be machined or cold worked. It also improves abrasion resistance. The cementite is formed into globules (spheroids) dispersed throughout the ferrite matrix.

Normalizing
Normalizing is similar to annealing, with a slight difference in the temperature and the rate of cooling. Steel is normalized by heating to about 60°C (110°F) above the austenite line and then cooling under natural convection.

The material is then air cooled. Normalizing produces a uniform, fine-grained microstructure. However,
since the rate of cooling is faster than that used for full annealing, shapes with varying thicknesses results in the normalized parts having less uniformity than could be achieved with annealing.

 Since structural plate has a uniform thickness, normalizing is an effective process and results in high fracture toughness of the material.

Hardening
Steel is hardened by heating it to a temperature above the transformation range and holding it until austenite is formed. The steel is then quenched (cooled rapidly) by plunging it into, or spraying it with, water, brine, or oil.

The rapid cooling “locks” the iron into a BCC structure, martensite, rather than allowing the transformation to the ferrite FCC structure. Martensite has a very hard and brittle structure.

Since the cooling occurs more rapidly at the surface of the material being hardened, the surface of the material is harder and more brittle than the interior of the element, creating nonhomogeneous characteristics. Due to the rapid cooling, hardening puts the steel in a state of strain.

This strain sometimes causes steel pieces with sharp angles or grooves to crack immediately after hardening. Thus, hardening must be followed by tempering.

Tempering
The predominance of martensite in quench-hardened steel results in an undesirable brittleness. Tempering is performed to improve ductility and toughness. Martensite is a somewhat unstable structure.

Heating causes carbon atoms to diffuse from martensite to produce a carbide precipitate and formation of ferrite and cementite.

After quenching, the steel is cooled to about 40°C then reheated by immersion in either oil or nitrate salts. The steel is maintained at the elevated temperature for about two hours and then cooled in still air.


Example of Heat Treatment
In the quest to produce high-strength low-alloy steels economically, the industry has developed specifications for several new steel products, such as A913. This steel is available with yield stresses ranging from 50,000 to 75,000 psi.

The superior properties of A913 steel are obtained by a quench self-tempering process. Following the last hot rolling pass for shaping, for which the temperature is typically 850°C (1600°F), an intense water-cooling spray is applied to the surface of the beam to quench (rapidly cool) the skin.

Cooling is interrupted before the core on the material is affected. The outer layers are then tempered as the internal heat of the beam flows to the surface. After the short cooling phase, the self-tempering temperature is 600°C (1100°F) (Bouchard and Axmann, 2000).

STEEL PRODUCTION BASICS AND TUTORIALS FOR STEELS USED IN CIVIL ENGINEERING PROJECTS

PRODUCTION OF STEEL BASIC INFORMATION
How Steels Are Made?


The overall process of steel production is shown in Figure 3.3. This process consists of the following three phases:

1. reducing iron ore to pig iron
2. refining pig iron (and scrap steel from recycling) to steel
3. forming the steel into products

Steel Production Process

The materials used to produce pig iron are coal, limestone, and iron ore. The coal, after transformation to coke, supplies carbon used to reduce iron oxides in the ore.

Limestone is used to help remove impurities. Prior to reduction, the concentration of iron in the ore is increased by crushing and soaking the ore.

The iron is magnetically extracted from the waste, and the extracted material is formed into pellets and fired. The processed ore contains about 65% iron.

Reduction of the ore to pig iron is accomplished in a blast furnace. The ore is heated in the presence of carbon. Oxygen in the ore reacts with carbon to form gases.

A flux is used to help remove impurities. The molten iron, with an excess of carbonin solution, collects at the bottom of the furnace. The impurities, slag, float on top of the molten pig iron.


The excess carbon, along with other impurities, must be removed to produce high-quality steel. Using the same refining process, scrap steel can be recycled. Two types of furnaces are used for refining pig iron to steel:

1. basic oxygen
2. electric arc

The basic oxygen furnaces remove excess carbon by reacting the carbon with oxygen to form gases. Lances circulate oxygen through the molten material. The process is continued until all impurities are removed and the desired carbon content is achieved.

Electric furnaces use an electric arc between carbon electrodes to melt and refine the steel. These plants require a tremendous amount of energy, and are used primarily to recycle scrap steel.

Electric furnaces are frequently used in minimills, which produce a limited range of products. In this process, molten steel is transferred to the ladle.

Alloying elements and additional agents can be added either in the furnace or the ladle. During the steel production process, oxygen may become dissolved in the liquid metal.

As the steel solidifies, the oxygen can combine with carbon to form carbon monoxide bubbles that are trapped in the steel and can act as initiation points for failure. Deoxidizing agents, such as aluminum, ferrosilicon and manganese, can eliminate the formation of the carbon monoxide bubbles.

Completely deoxidized steels are known as killed steels. Steels that are generally killed include:

■ Those with a carbon content greater than 0.25%
■ All forging grades of steels
■ Structural steels with carbon content between 0.15 and 0.25 percent
■ Some special steel in the lower carbon ranges

Regardless of the refining process, the molten steel, with the desired chemical composition, is then either cast into ingots (large blocks of steel) or cast continuously into a desired shape. Continuous casting with hot rolling is becoming the standard production method, since it is more energy efficient than casting ingots, as the ingots
must be reheated prior to shaping the steel into the final product.

Cold-formed steel is produced from sheets or coils of hot rolled steel which is formed into shape either through press-braking blanks sheared from sheets or coils, or more commonly, by rollforming the steel through a series of dies. No heat is required to form the shapes (unlike hot-rolled steel), and thus the name cold-formed steel.

Cold-formed steel members and other products are thinner, lighter, and easier to produce, and typically
cost less than their hot-rolled counterparts (Elhajj, 2001).